1. Field of the Invention
This invention relates to the manufacture and use of non-woven thermoplastic veils for use in complex and diverse high performance composite applications. In preferred embodiments, this invention relates to a resin-soluble thermoplastic polymer formed into a non-woven veil fabric or random mat for incorporation into high performance composite manufacturing applications to aid part manufacturing and improve desired properties.
2. Description of the Related Art
Fiber-reinforced resin matrix composites are widely accepted for use as high strength low weight engineering materials to replace metals in aircraft structural applications and the like. These curable compositions may be made by laminating prepregs comprising high strength fibers, such as glass, quartz, graphite (carbon), boron, aramid or the like impregnated with an epoxy resin matrix. Important properties of such composites are high strength and stiffness with reduced weight.
Curable compositions comprising a blend of polymer resins and reinforcing fibers are characterized by individual physical and chemical properties of the constituent polymer resins and fibers, whereby compositions may be selected for a specific use. Typically, therefore a thermoset resin component is present which confers high solvent resistance, thermal cycling resistance, etc. In addition, a thermoplast resin component may be present to confer higher levels of toughness, fire resistance, etc, and reinforcing fibers to confer high levels of stiffness, strength, etc.
Composites are traditionally made using curable compositions known as prepregs made up of fiber reinforcements or structural fabrics impregnated with a resin matrix. Sheets of prepreg may be cut to size for laying up and molding and laminating in the construction of a given composite structure. Prepreg properties and resulting part quality can be controlled to manipulate resulting part properties such as toughness, strength, flexibility and the like.
Prepregs traditionally contain epoxy resins impregnated on fiber reinforcements. Should the resulting part require enhanced properties such as additional toughness, additives such as thermoplastics may be added to the epoxy resin. However, use of prepregs results in certain disadvantages including labor costs, difficulty in forming complex shaped parts, controlling toughened locations, and increased manufacturing costs due to the use of automated tape laying or fiber placement equipment and autoclaves for curing.
Recently there has been an emergence of an alternative technology for manufacturing composite parts, which technology is generally referred to as resin infusion. This approach differs from that of conventional prepreg technology in that dry structural reinforcement fibers are arranged in a mold as a preform, which consists of one or more layers or plies of dry oriented structural reinforcement fiber material assembled in a stack. Then the preform is then injected or infused directly in-situ with the resin matrix.
Resin infusion is a generic term which covers processing techniques such as Resin Transfer Molding (RTM), Liquid Resin Infusion (LRI), Vacuum Assisted Resin Transfer Molding (VARTM), Resin Infusion with Flexible Tooling (RIFT), Vacuum Assisted Resin Infusion (VARI), Resin Film Infusion (RFI), Controlled Atmospheric Pressure Resin Infusion (CAPRI), VAP (Vacuum Assisted Process) and Single Line Injection (SLI).
The potential benefits that resin infusion has to offer over that of a conventional prepreg route are reduced scrap, reduced lay-up time, reduced capital cost, a non-dependence on tack and drape and increased shelf life properties. In practice, the use of resin infusion technology finds its greatest use in specialized operations requiring complex composite structures, localized toughening, and very large structures, such as in aircraft wings and fuselages, marine or wind applications.
When the preform is placed in a mold, the layers or plies are typically held in place, stabilized and compacted/debulked, by stitching, stapling or bonding using binders and tackyfiers. These operations maintain the orientation of the fibers and to stabilize the preform in order to maintain the geometry and the dimensions of the preform and to prevent fraying or the pulling apart of the dry perform during storage, transport and handling.
The preform may then be carefully cut outside of the stitching or stapling to a desired shape. The preform is then placed in a mold and resin injected to impregnate the fabric. The infused preform is then cured by ordinary and accepted procedures to provide a finished composite structure.
However, stitching and stapling for preform stabilization are typically limited as the preform cannot be shaped to conform to a complex structure's contour without disturbing the stitching or stapling.
One means of overcoming the stitching problem has been through the use of weaving a thermoplastic into the structural reinforcement fibers, which thermoplastic will melt when slightly heated to stabilize the preform, and then will melt into the epoxy resin matrix during cure. See U.S. Pat. No. 4,741,073, for example. However, this practice is limited in the amount of thermoplastic that can be added, requires high processing temperatures to ensure complete melting of the thermoplastic into the epoxy resin matrix.
While resin infusion technology is promising, composites which have high impact requirements usually contain thermoplastic toughening agents and it is difficult to add a thermoplastic toughening agent to an injectable resin because thermoplastic toughening agents possess such high molecular weight that they greatly increase the viscosity of the resin. Therefore, only small amounts of a thermoplastic toughening agent can be added to the resin of these resin infusion systems.
A potential way to efficiently provide thermoplastic toughening agents for a resin infusion system is to not add the thermoplastic to the resin, and to introduce it in some other way to the preform. In the case of resin infusion technology, one means of overcoming this limitation has been by introducing the thermoplastic polymer by weaving it directly into the carbon fibers.
EP 392939 discloses a method of preparing a traditional prepreg with reinforcing fibers co-woven with thermoplastic fibers which melt. These systems do not however attempt to introduce an additional resin matrix into the prepreg, as in resin infusion systems, and typically employ very high molecular weight thermoplastic polymers, which require excessively high temperature and pressure to melt.
One early attempt at incorporating a thermoplastic fiber into carbon fiber fabric for resin infusion is disclosed in U.S. application Ser. No. 10/381,540. This application discloses the use of a thermoplastic fiber woven within the structural reinforcement fiber for use in a resin infusion preform system. However, this application discloses that the thermoplastic fibers remain in the final composite, after curing of component, i.e., the thermoplastic fibers do not dissolve in the resin.
A related attempt to overcome these issues in resin infusion technology is to provide a preform which introduces a flexible thermoplastic polymer fiber with the reinforcing fibers by weaving the thermoplastic polymer fiber with the reinforcing fibers. The flexible thermoplastic polymer fiber is in a solid phase and adapted to undergo at least partial phase transition to fluid phase on contact with a resin matrix component of the curable composition at a temperature that is less than cure temperature for the curable composition and which is less than the melting temperature of the flexible thermoplastic polymer fiber. See published US Patent Application No. US 20040041128 assigned to Cytec Technologies, Inc. and published Mar. 4, 2004.
While this method of co-weaving a thermoplastic fiber with the reinforcement fibers overcomes many of the problems of traditional resin infusion technology, it does not provide the capability of localized toughening. The thermoplastic fibers are woven throughout the reinforcement fibers and such that there is an even distribution of fibers and resulting even concentration of thermoplastic throughout the composite part. Therefore, this method has limited flexibility in the amount of added thermoplastic polymer that may be incorporated into the composite part. There is no provision in this co-weaving technology to concentrate additional amounts of thermoplastic polymer for increased toughening at a particular location of the composite part.
Additionally, reinforcement fiber fabrics co-woven with thermoplastic fibers concentrate the thermoplastic toughening agent predominantly within the same plane as the reinforcement fibers rather than between layers where it may be more preferred. The toughening agent is preferred to be primarily situated between structural reinforcement fiber layers, rather than within, to carry the higher stresses incurred between the reinforcement fiber layers of the final composite.
Moreover, the thermoplastic fibers are not practically able to be added to unidirectional tape due to the high tension required to manufacture unidirectional carbon fiber tape.
The incorporation of the thermoplastic toughening agent as a fiber co-woven into the structural reinforcement fiber further led to some difficulties in controlling amount of thermoplastic toughening agent in the resin. This control is preferred in order to match data for current resin formulations containing precise amounts of thermoplastic toughening agent. In order to obtain the appropriate amount of thermoplastic polymer, the number of thermoplastic fibers and uniformity of distribution throughout the structural reinforcement preform had to be controlled. The number, size and placement of the thermoplastic fibers within the preform had to be controlled to allow for the uniform distribution of the thermoplastic polymer to result in the duplication of current toughened resin formulations.
The co-weaving method also results in increased manufacturing costs due to the requirement of spinning the thermoplastic polymer into a fiber capable of being woven with the structural reinforcement fibers. Moreover, the added complexity of co-weaving a dissimilar material such as a thermoplastic fiber with the structural reinforcement fibers increased manufacturing costs.
Additionally, co-weaving of the thermoplastic fiber disturbed the arrangement and straightness of the structural reinforcement fibers, thus reducing in-plane mechanical properties and did not afford the interlaminar toughening that was desired of composite systems. In other words, the thermoplastic polymer is preferred to be concentrated more highly between the structural reinforcement fiber layers rather than within.
A modified attempt to overcome these shortcomings of co-woven toughened resin infusion systems has been to interpose thermoplastic interleaf layers between reinforcement structural fiber layers. See U.S. Pat. Nos. 6,437,080B1, 5,288,547, EP 1 473,132 A2 and EP 0327,142 A. However, this interleaf toughening material does not dissolve into the resin, but rather melts at the higher temperatures attained during cure providing limited diffusion and requiring higher temperatures and cure times to advance the melting and diffusion.
Melting interleaf layers are limited in that it is difficult to manufacture quality components by RIFT or VARTM, out of the autoclave, without the benefit of high temperatures and cure times needed to advance the melting and diffusion in these systems. Curing with vacuum only or with no pressure causes the components to have very high void content thus, leading to poor mechanical properties.
Melting interleaf systems are further limited in that the lack of uniform and complete dissolution creates interfaces between the resin and reinforcement fibers. This interface can in turn decrease composite resistance to fluids and cause a reduction in hot/wet mechanical properties. Furthermore, the lack of complete dissolution of the thermoplastic and the resin means that no synergy between the properties of the two chemical species can be obtained; for example the resin will maintain its brittleness and the thermoplastic will remain sensitive to solvents.
Other means of incorporating thermoplastic tougheners into resin infusion systems include powder coating the dry fibers or the fabric with a particulate thermoplastic material such that when the dry fibers or the fabric are laid up, slight heating of the fibers melts the particulate thermoplastics and fuses and stabilizes the plies. Additionally, the thermoplastic particles melt at the cure temperature and diffuse into the resin system, toughening the matrix. However, melting and diffusion of these thermoplastics into the resin matrix also requires the high temperature curing processes and is limited in the amount and extent of diffusion of the thermoplastic. See U.S. Pat. No. 5,057,353 which uses thermoplastic particles on the surface of the resin infused prepregs to add toughening properties. This system is an epoxy resin impregnated prepreg rather than a coated dry fiber applicable for resin infusion. Furthermore, the thermoplastic particles are intended to melt upon curing and do not dissolve prior to attaining higher cure temperatures necessary to melt them. Another disadvantage of particle toughening is that during resin infusion, the particles can be washed away by the resin flow and can uncontrollably agglomerate at undesirable locations. This causes the mechanical properties of the composites to be not uniform and may cause undesirable voidage and porosity due to varying viscosities of the resin causing a non-homogeneous flow front.
Other methods of incorporating thermoplastic particles or fibers into reinforcement structural fibers are disclosed in EP 0842038 B1; WO 03/038175 A1; JP 2000119952 A2 and U.S. Pat. No. 6,060,147.
However, localized toughening of composites remains difficult using these techniques during manufacture because the toughening agent is uniformly coated onto the reinforcement fibers and cannot be controllably increased in any particular area of the part that may require increased toughness.
It has been proposed to use hybrid matrix thermosetting resins including a high molecular weight thermoplastic polymer, as a particulate dispersion as disclosed for example in GB-A-2060490, or as a particulate coating or film interleave of the fiber-reinforced matrix resin prepregs as disclosed in U.S. Pat. No. 5,057,353. Nevertheless dispersion is typically poor due to difficulty in controlling distribution of particles and uniformity of particle size which can influence rate and degree of melting, and the barrier effect of a continuous film during resin infusion.
U.S. Pat. No. 5,288,547 discloses prepregs for curable compositions comprising a porous thermoplastic polymer membrane interleave. The membrane is laid up against a sheet of reinforcing fiber and melting at elevated temperature and pressure to impregnate the fibers; alternatively prepreg is laid up with membrane between and melted to impregnate prior to curing to form a composite part; alternatively the membrane proposed for RTM application is laid up between layers of dry fiber in a mold, melted to impregnate, and liquid resin injected into the mold.
While these technologies go some way to alleviating the problems associated with thermoplastic toughened resin infusion systems, there is still a need for a more versatile solution with more flexibility and control of nature and amount of toughening agent and increased performance properties. Specifically, the need for an improved means of incorporating and controlling the amount of thermoplastic toughener in a resin infusion system remains unsatisfied. Indeed, there remains a need to introduce greater amounts of thermoplastic polymers for toughening into the system.